1. Field of the Invention
The invention relates to cranes in general, and particularly concerns a crane with telescopic jibs and with a head jib articulated thereon, whose one jib is provided with one or several catch hooks with abutting surfaces which keep the head jib bearing on the telescopic jib in the rest position of the crane, and which are detachable.
2. Description of the Prior Art
Cranes are known, where one or several catch hooks are generally arranged on the head jib, whose swivel axis extends transversely to the major axis of the jib. The catch hooks are kept by springs in their locking position and are provided with so-called rip cords, by which the catch hooks can be loosened. For dismantling or stowing the implements, the jibs are lifted close to their vertical positions. The head jib, which is articulated on the front end of the ground jib for rotation about a horizontal axis, swings and finally opens the catch hooks when bearing on the ground jib, which then snap in and keep the head jib bearing on the ground jib. For rigging or erecting the crane, the rip cord is pulled, so that it or the catch hooks release the movement of the head jib and they can be brought into the head position. In this design, the two jib parts must always be brought into the same position with respect to each other during the above described dismantling, so that the catch hooks are in the correct position relative to the opposite pieces or catch members on the other jib part.
A crane with telescopic jibs is also known where the locking mechanisms consists of projections and counterparts arranged on the head jib and on the ground jib, and on the other hand, release each other during the extension.
To this end, tongues provided with slots are arranged on the head jib, and on the other hand, projections are provided on the ground jib. Care must be taken that, when the extensible jib portion of the head jib and the ground jib are in a certain position to each other during the retraction; the projections are in the proper position with respect to the slots in the following displacement of the two jibs. This requires auxiliary means. These consist, on the one hand of a cable guide for the hoisting cable arranged on the ground jib. The hoisting cable is first looped around this cable guide. Furthermore the hoisting cable must be anchored on the supporting frame of the jib on which the hoisting cable can be secured by an operator with the head jib lowered, after it has been looped around the cable guide. The hoisting cable of the head jib, laid and anchored this way, can then be pulled to the ground jib. Then the insertable end of the ground jib is retracted. A guide is provided for the engagement of the locking means, which is arranged between the ground jibs and the head jib, and which ensures the necessary proper position of the two jib parts to each other. This guide consists of a guide rod on the ground jib, which engages the groove of the cable pulley at the front end of the head jib in the rest position of the latter (U.S. Pat. No. 3,366,250).